After upgrading the 1800 engine in my 124 AS Spider with a GC prepped head, twin IDF 44's and a GC IIB intake cam on it's way to be installed this winter I felt that I should replace the original gear box with the sturdier 131 five speed type. I found one and set to work at replacing the seals and gaskets. Stupidly enough during this work debris found it's way into the rear main shaft bearing and I sourced a new one.

During assembly of the tail housing I noticed something odd that I did not realise when disassembling:

The main shaft rear ball bearing has been press fitted into the tail housing of the gear box. There is no collar or chamber that defines the exact location of the bearing in the bore of the tail housing.

I may be wrong but to me this is unusual because the both bearings of the main shaft will be subject to axial load if the position of the bearing in the tail housing is not exactly right. The 124 gear box has a roller bearing on the rear of the main shaft but the tail housing of that box has a sliding fit for the bearing which facilitates for axial expansion of the shaft.

Resorting to the Fiat 131 manual (Haynes as I haven't been able to locate an original Fiat one to this date) offered no insight as the rear main shaft construction seems to be different to this manual from what I have in my shop:
- Reverse gear and speedometer gears locked into place by snap rings instead of the spider coupling and nut (see photo B and C versus photo E and F);
- Rear main shaft plain bearing instead of a ball bearing (see photo D versus photo F).

This manual states to be covering the 131 Mirafiori Sport my gear box originated from but does note describe the remote gear shift mechanism it came with. It may be possible that the sport box is different from the normal saloon box.

In order to move on I would appreciate if any of the raiders has an answer to my following questions:
1: Can anybody post a cross section from the OE Fiat 131 manual if available?
2: Has anybody experience is replacing this bearing and is it indeed meant to be press fit?
3: If so how does one determine the exact to be position of the bearing in the tail housing without significant measurement error?
4: I'm considering widening the bore in the tail housing to sliding fit of the bearing. Would this incur potential problems?

Problem solved::
- widened the bore in the tail housing with a honing tool until sliding fit;
- Fixated the outer race of the bearing with Loctite 648 to prevent it from rotating. I had to tighten and torque the tail housing to gearbox bolts as well as the spider coupling nut within five minutes as this is the working time before curing.